1. Field of the Invention
This invention deals with isocyanate synthesis. More specifically, it discloses a non-phosgene process for the preparation of hexamethylene diisocyanate and isomeric diisocyanate.
2. Description of the Prior Art
The commercial production of hexamethylene 1,6-diisocyanate is based on the phosgenation of hexamethylenediamine to form hexamethylene bis(carbamyl chloride) and its thermal cleavage into hexamethylene diisocyanate and hydrogen chloride. Aside from serious environmental and safety problems involved with the use of phosgene, this process also has further disadvantages. For example, the production of hexamethylene diisocyanate results in rather poor volume-time yields. In addition to hexamethylene diisocyanate, several by-products result, of which the most important, 6-chlorohexyl isocyanate, has the further disadvantage that a relatively elaborate procedure is required to separate it from the hexamethylene diisocyanate.
A problem with this process lies in the high conversion of chlorine via phosgene and carbamyl chloride in hydrogen chloride, the toxicity of phosgene, as well as the corrosiveness of the reaction mixture and the low stability of the solvents that are generally used. There have been numerous attempts to prepare isocyanates, preferably aromatic di- and/or polyisocyanates, without using phosgene.
In EP-A 28 338 aromatic di- and/or polyisocyanates are prepared in a two-step process, whereby primary aromatic di- and/or polyamines are reacted in the first step with O-alkylcarbamic acid esters in the presence or absence of catalysts and, in some cases, urea and alcohol to form aryldi- and/or polyurethanes, and the ammonia formed in this process may, in some cases, be separated off, and the aryldi- and/or polyurethanes that were obtained can be converted into aromatic di- and/or polyisocyanates in the second reaction step by means of thermal cleavage. In this way, aromatic di- and/or polyisocyanates can be prepared with high yields and without using phosgene.
DE OS No. 31 08 990 describes the preparation of isophorone diisocyanate through the thermal cleavage under pressure of 3-ethoxycarbonylaminomethyl-3,5,5-trimethyl-1-ethoxycarbonylaminocyclohexa ne in the presence of dibenzyltoluene as a solvent and a catalyst mixture of toluene methylsulfonate and diphenyl tin dichloride. No information is given on obtaining the initial components, isolating and purifying the initial components, possible recovery of the solvent, or on the catalyst mixture. Thus, calculations of the economic feasibility of the process cannot be made.
Based on the information in German Patent Application Nos. P 32 27 748.2, P 32 48 018.0 and P 31 42 627.1 hexamethylene dialkyl urethanes can be decomposed without using catalysts or in a fluidized bed containing carbon into hexamethylene diisocyanate and alcohols. However, this process cannot be used to obtain hexamethylene diisocyanate yields greater than 90 percent, since the cleavage products partially recombine. The yield can be further decreased by the subsequent purification distillation that is needed for the hexamethylene diisocyanate.